The present invention relates to an acousto-optic tunable filter (hereinafter "AOTF"), and more particularly, to a method and apparatus for improving the bandpass characteristic of an acousto-optic filter. Acousto-optic tunable filters can comprise a variety of designs such as collinear, non-collinear and acousto-optic dispersive light filter (hereinafter "AODLF"). The design and operation of an AODLF is described in U.S. Pat. No. 4,639,092 and U.S. Pat. No. 4,653,869, both assigned to the same assignee of the subject application, and which are hereby incorporated by reference.
In general, the transmission characteristic of an acousto-optic filter is defined by: ##EQU1## where T.sub.0 is the peak value of the transmission, .lambda..sub.0 is the center wavelength, "a" is the 4 dB bandwidth, and ##EQU2## An acousto-optic light filter depends upon optical diffraction to achieve filtering. Thus, the transmission of the filter refers to the diffracted beam, and the undiffracted beam passes through the filter and is eliminated by, for example, polarizers or spatial discrimination.
The solid line in FIG. 1 represents a plot of the function expressed by equation (1). The solid waveform in FIG. 1 has a bandwidth of "a" (at the 4 dB point from its peak). The amplitude of the solid waveform sidebands diminishes as shown in FIG. 1 and as defined by equation 1. The zero points of the transmission occur at .vertline..lambda.-.lambda..sub.0 .vertline.=na where n is an integer. The maxima of the transmission are spaced nearly halfway between the zero point nodes.